(i) Field of the Invention
This invention relates to a process of producing value-added bioproducts from a lignocellulosic biomass.
Lignocellulosic biomass in the invention refers to any type of plant biomass that is composed of cellulose, hemicellulose and lignin, typically in amounts of 30-55%, by weight, cellulose; 15-35%, by weight, hemicellulose; and 5-31%, %, by weight, lignin.
(ii) Description of Prior Art
Cellulose is the main structural constituent in plant cell walls. Cellulose is a carbohydrate comprising long chains of the six carbon sugar glucose that are connected end to end linearly as a polymer through β-O-4 glucosidic bonds. The long-chain cellulose is linked together by hydrogen and van der Waals bonds, which cause the cellulose to be packed into microfibrils. Hemicellulose and lignin cover the microfibrils.
Hemicellulose is a complex carbohydrate structure that consists of different polymers like pentoses (like xylose and arabinose), hexoses (like mannose, glucose, and galactose), and sugar acids. The dominant component of hemicellulose from hardwood and softwood is xylan and glucomannan, respectively.
Lignin is a complex, large heteropolymer consisting of three different phenylpropane units (coniferyl, coumaryl, and sinapyl alcohols) held together by different kinds of linkages.
Bioconversion of lignocellulosic materials to biofuel and bioproducts is a multistep process which involves the breakdown of the feedstocks followed by hydrolysis and extraction of various components. For example, bioconversion of lignocellulosic materials to biofuel (e.g. ethanol) typically consists of four main process steps: pretreatment, enzyme hydrolysis, fermentation and products recovery. There are also processes designed to extract specific components from biomass feedstock. These processes also employ a pretreatment step for the purpose of reducing the particle size of the biomass and creating large surface area for downstream processes. Pretreatment has a major impact on bioconversion process efficiency and economics. Many pretreatment techniques/methods have been developed for lignocellulosic materials using mechanical force, steam, acid, alkaline or biological agents or a combination of two or more thereof.
Mechanical refining has attracted increasing attention recently as a pretreatment for lignocellulosic biomass. Mechanical refining is an industrial process employed to produce mechanical pulp where the biomass raw materials are separated into fibres by a combination of heat and mechanical force. Several variations of mechanical pulping process are employed on an industrial scale, including refiner mechanical pulping (RMP), thermomechanical pulping (TMP), chemithermomechanical pulping (CTMP) and chemimechanical pulping (CMP). One of the major disadvantages of using mechanical pulping for a papermaking process is, the high energy consumption associated with mechanical pulping, producing one ton of mechanical pulp typically requires up to 4000 kWh of electricity. The bulk of the energy consumed during refining is to develop pulp fibre while only a small fraction of the energy is directed to the actual fibre separation.
Pretreating lignocellulosic biomass using a refiner has been a subject of several recent studies. Colin et al in WO 2008/131229 A1 disclosed a method of processing lignocellulosic biomass using a combined thermochemical pretreatment and mechanical refining. The refining pretreatment was carried out at a temperature between 165° C. and 220° C., and a pressure 100 psig to 700 psig (7-48 bars or 700 to 4800 kPa), for a duration of 5 seconds to 15 minutes. The focus of Colin et al was to improve the glucose yield in a subsequent cellulose hydrolysis, for ethanol production with little discussion on the utilization of hemicellulose and lignin content of the lignocellulosic biomass. Another recent research carried out by Zhu et al (Bioresources Technology, 100 (2009), 2411-2418) incorporates disk refining as a post-treatment after sulphite pulping (pretreatment) to reduce particle size of the substrate and enhance hydrolysis efficiency. A significant amount of chemicals, including 8-10% bisulfite and 1.8-3.7% sulfuric acid, were applied during sulphite pretreatment which are expected to significantly alter the nature of lignin. Pschorn et al. US 2008/0277082 A1 disclosed a high pressure compressor which is converted from a mechanical refiner and can be used for treating the biomass by a steam explosion method.